Printed circuit boards are tested prior to distribution in order to determine whether they have any electrical defects. This is generally accomplished by connecting a circuit board to be tested to a test system by means of a test fixture having a plurality of probes for contacting certain points on the back of the board.
The circuit boards are generally delivered to a test area in a container which is commonly referred to as a "tote box." The tote box has a series of slots, each of which contains one circuit board. The dimensions of each slot are sufficiently large to accommodate a variety of board sizes and to facilitate loading of the tote box. Similarly, the space between each slot is sufficient to accommodate the components which extend above the surface of the board. Thus, the circuit boards are relatively loosely fitted within the tote
A test station is usually operated by a person standing within reach of the tote box and the test fixture of a test system. The operator repetitively reaches into the tote box for a circuit board to be tested, places it on the test fixture and waits for the test to be completed. Upon completion of the test, the board is removed from the test fixture and placed with known good boards or routed to a repair area.
Various attempts have been made to replace the human operator with a robot. However, a number of problems have been encountered in creating systems which can pick up the circuit boards from tote boxes, transport them without dropping them, accurately place them on a test fixture and finally, pick them up again from the test fixture after completion of the test, all without inflicting any damage to the board or the components found thereon. To begin with, picking up a circuit board from a tote box is quite different than picking one up from a gasketed test fixture. This is due to the fact that the location of circuit boards within the slots of tote boxes is not as accurate as their location when properly placed upon a gasketed test fixture. The variation in location within a tote box is on the order of plus or minus 30 mils. For circuit boards properly located upon test fixtures, this variation runs only about 4 mils. Accordingly, a suitable robotic hand must meet different sets of requirements in order to be able to pick up and deposit a circuit board in these two different situations. A complicating factor concerns the gasket configuration typically found on test fixtures. It is normally provided with a recess into which the board is placed, thereby at least partially blocking access to the edges of the board. Still another problem, at least with some boards, is that they are somewhat flexible. As a result, they can be easily dropped during transportation by a robot when they flex under the inertial forces experienced with today's high speed robot systems. A final problem is that most circuit boards offer very little edge space that is free of components or electrical leads. Thus, it becomes quite difficult to grip a board by its edges without causing damage.
Prior devices have sought to overcome these problems in a number of ways. One device utilizes a large number of vacuum cups which grasp and hold the loaded circuit boards by means of suction. The vacuum cups are arranged on the hand so as to correspond with components having a flat surface in order to obtain a secure grip on the board. Due to the lack of large flat surface areas on most printed circuit boards, and also due to the movement of the robot itself, which tends to dislodge the boards, a large number of vacuum cups are generally required. Also, since the vacuum cups must be in a pattern corresponding to that of the components having flat surfaces, a different hand member has to be used for each different type of circuit board to be tested. The need to match hand members to the circuit boards being tested is not only time consuming, but also expensive, and therefore, preferably avoided. While the vacuum cups themselves do not generally damage the components, the cups can attach to the circuit board at improper locations due to the varying positions of the circuit boards in the tote boxes. This results in less than adequate suction and it is likely that the board will be dropped during transportation to or from the test fixture. Aside from the damage which can be caused to the board if it is dropped, most systems will automatically stop when any such irregularity occurs, thus requiring human intervention to cure the difficulty and restart the procedure.
Another approach which has been used for transporting loaded circuit boards utilizes a pair of grooved parallel bars. The circuit board is positioned between the bars after which the bars are moved together, thereby wedging the circuit board between the bars and within the grooves. One drawback of this approach is that the circuit board must have two parallel edges and the edges must be free from components so as not to be damaged when the bars grasp the board. Other difficulties are encountered when the circuit boards being tested are flexible and cannot be secured simply by wedging them between the bars or when the circuit boards, which can vary in width by about 30 mils, are of a width that does not correspond to the spacing of the bars. Finally, it has been found that this approach tends to damage the test fixture gasket or loosen the connection between the gasket and the test fixture. Alternatively this approach would require the incorporation of complex and expensive structures within the fixture to lift the board from the fixture in order to allow it to be grasped by the parallel bars.
These and other problems are solved by the current invention. The robotic hand of this invention is capable of reaching into the relatively narrow confines of a tote box slot and gripping a loaded circuit board previously placed therein with a degree of accuracy typical of current practice. Also, the robotic hand of this invention can grip a circuit board at selected spots along its edges without causing any damage to the board or any of the components found thereon. The resulting grip is firm enough and secure enough to withstand the inertial forces encountered in a high speed robotic system. Finally, the device to be described herein is suitable for picking up a circuit board from a recessed position within a test fixture gasket without damaging the gasket in any way or loosening its connection with the test fixture or requiring extra expensive lift devices on the various test fixtures to be utilized. These and other attributes of the invention will become apparent in the detailed description which follows.
It is, therefore, an object of this invention to provide a robotic hand suitable for picking up and transporting a loaded circuit board by making minimal contact with its edges.
It is a further object of this invention to provide a robotic hand of the type described which is suitable for reaching into a typical tote box and picking up a loaded circuit board previously placed therein and ultimately replacing that board in either the same or another tote box with a degree of accuracy in accordance with present practices.
It is a further object of this invention to provide a robotic hand of the type described suitable for accurately placing a circuit board on a test fixture, releasing the circuit board and withdrawing the hand without causing damage to the gasket typically found on test fixtures.
It is a further object of this invention to provide a robotic hand of the type described suitable for picking up a circuit board from a test fixture without requiring modification of test fixture or causing damage to the test fixture gasket.
It is a further object of this invention to provide a robotic hand of the type described capable of gripping a flexible circuit board with sufficient strength so as not to drop the board when it flexes under the inertial forces experienced in a high speed robotic system.
Briefly described, the above objects are accomplished with a hand member having at least two and preferably three opposing pincer units.
In the preferred embodiment, two of the pincer units are designed to engage the same edge of the circuit board and lie along a predetermined reference line. These two pincer units are laterally adjustable relative to each other so as to engage clear spots on the edge of the board. A pincer unit adjacent the opposite edge of the board is longitudinally adjustable to accommodate boards of various sizes. Each pincer unit has a thumb and finger set operated between a pinching state where the gap between them is relatively closed and a releasing state wherein the gap is relatively opened. In addition, since these pincer units depend from the hand, the circuit board can be held in a spaced-apart relationship with respect to the palm which allows room for the various circuit board components. The opposing pincers are also mounted so as to allow the distances between the pincers to be varied. This permits the pincers to squeeze the circuit board between themselves while pinching its edges. This dual securing aspect of the invention allows for a sufficiently firm grip of the circuit board without damaging any of the components. In addition, since the fingers extend outwardly from the hand, they can be used to depress the gaskets along a selected short section of the boards perimeter and can then be extended beneath the circuit board for removing it from the test fixture without damaging the gasket.
The method of this invention involves the steps of bracketing a pair of opposing edges of a circuit board with pincer units having variable pinching gaps, squeezing the board between the pincer units by moving the pincer units toward one another and finally, pinching the edges of the circuit board.